Microwave heating cavity with a venetian blind mode stirrer



July 21,1970 J. R. WHITE 3,521,019

MICROWAVE HEATING CAVITY WITH A VENETIAN BLIND MODE STIRRER Filed Feb.19, 1968 MICROWAVE FIG. 2 GENERATOR A 9 ,ggfwr g X} 271 ll l\I I II I l1 1 1 Ll I l 1;] LI ll. lllllll a I H H I I ls I I I I H IIIIIIIIIIIHHUnI I l lggnr v l I I I,

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Y JEROME WHITE Wfiih ATTORNEY United States Patent 3,521,019 MICROWAVEHEATING CAVITY WITH A VENETIAN BLIND MODE STIRRER Jerome R. White, SanCarlos, Califi, assignor to Varian Associates, Palo Alto, Calif., acorporation of California Filed Feb. 19, 1968, Ser. No. 706,482 Int. Cl.H05b 9/06 US. Cl. 219-1055 13 Claims ABSTRACT OF THE DISCLOSURE Amultimode rectangular cavity is excited by a microwave generator coupledby a waveguide feed to one side of the cavity. A plurality of conductiveslats, each about one half wavelength wide, are rotatably mounted infront of a wall of the cavity opposite the side of the cavity at whichthe waveguide feed is located. The slats are mounted parallel spacedapart in a plane so that the closest possible approach between adjacentslats is less than one quarter wavelength. The slats are spaced from thewall which they front so that the closest possible approach between the*wall and slats is less than one quarter wavelength. A drive motor iscoupled by a transmission to synchronously rotate the slats.

BACKGROUND OF THE INVENTION In subjecting materials to microwave energy,it is the practice to expose them to an electromagnetic fieldestablished in anexcited microwave resonating device or cavity. Tosubject the materials to a time-averaged uniform electrornagnetic 'fieldintensity, especially when in forms having a dimension on the order ofthe free space wavelength of the applied energy, generally, themicrowave cavity is constructed so as to support a multiplicity of fieldintensity distributions, or mode patterns. Suhc a device is commonlyreferred to as a multimode mircrowave cavity. By causing the fieldintensity distribution to be changed, i.e., mode stirring, thetime-averaged energy available to all portions of the microwave cavityis made more uniform thereby effecting a uniform application ofelectromagnetic energy to the material.

Heretofore, various mehcanical and electronic techniques have beenemployed to effect the changing of field intensity distribution.Electronic mode stirring techniques involve either modulating thefrequency of the microwave generator or providing multiple inputs to themicrowave cavity. Such electronic techniques involve either complicatedmicrowave generators or critical placement of the input waveguide feedrelative to the microwave cavity.

With respect to mechanical mode stirrers, basically there are two types;moving antenna feeds, and those which effect a change in the geometricspace of the microwave cavity as seen by electromagnetic fieldestablished therein. For practical reasons, of the two types, the movingantenna feed has proven the least suitable. This is because thewaveguide plumbing feeding the moving antenna requires an exceedinglycomplex construction in order to prevent damaging reflected microwaveenergy from reaching themicrowave generator. Furthermore, movingantennas are effective only in those cases where the material beingsubjected to the electromagnetic fields are extremely absorptive of theelectromagnetic energy.

Changes in the geometric space seen by the electromagn'etic field hasbeen accomplished a number of ways. For example, microwave cavities havebeen constructed with deformable walls which, when moved, effect achange in the geometric space. Such devices have the advantage ofcausing a real change in geometric space of cavity, hence are able toeffect changes in the field intensity distribution throughout thecavity. However, such devices have the disadvantage of beingstructurally complex, requiring complex prime movers for the deformablewall, not being able to accomplish rapid changing of the fielddistribution in the microwave cavity, and causing undesirable microwaveenergy reflections which are detected by the power meter monitoring theoutput of the microwave generator and the normal power reflected fromthe load as variations in the net power delivered to the load. Theaforementioned reflections make the evaluation of the time-averagedpower delivered to the load very diflicult.

Various conductive members mounted for movement within the microwavecavity also have been employed to effect a variation in the geometricseen by the electromagnetic field. These are generally classified asaxially reciprocating translatable mode stirrers, and rotatably mountedreciprocating and revolving mode stirrers. All have certain advantagesover the deformable wall-type mode stirrers, particularly, being ofsimple relative construction and being able to effect rapid changes inthe field intensity distribution. However, the rotatably mounted modestirrers are superior to the axially reciprocating translatable modestirrers in that they can cause a more rapid change of the fielddistribution and they require less complex prime mover and transmissionsystems. Unfortunately, the prior art rotatably mounted mode stirrersgenerally produce only localized changes in the field intensitydistribution. Hence, in large volume microwave cavities, the fieldintensity distributions in regions of the cavity remote from therotatably mounted mode stirrer will not significantly be changed. Toeffect the desired changes in the field intensity distribution of largevolume cavities, it has been the practice to locate several rotatablymounted mode stirrers at various locations in the cavity, generally, atdifferent cavity walls. When several rotatably mounted mode stirrers areused, it is necessary either to use a separate drive motor for each modestirrer or to employ a very complicated transmission system to link eachof the mode stirrers to a single drive motor.

Therefore, a great need exists of a mode stirrer which has the advantagecharacterizing the deformable wall-type mode stirrer of being able toeffect from a single location in a cavity changes in the field intensitydistribution throughout the volume of a cavity regardless of its sizewhile also having the advantages characterizing the rotatably mountedtypes of mode stirrers of a simplified construction and of being able toaccomplish rapid shifting of the field intensity distribution.

SUMMARY OF THE INVENTION The present invention relates to multimodemicrowave cavities for subjecting materials to microwave energy. Moreparticularly, it relates to such a multimode microwave cavity includinga rotatably mounted mode stirrer assembly which effects a change in thedistribution in the field intensity throughout the volume of the cavityby simulating a change in the geometric space of the cavity as would beproduced by deforming or moving a wall of the cavity.

Accordingly, it is an object of the present invention to establish amore uniform time-averaged electromagnetic field distribution throughouta microwave cavity.

More particularly, it is an object of this invention to establish a moreuniform time-averaged electromagnetic field distribution throughout amicrowave cavity by operating a rotatably mounted mode stirrer assemblylocated in a single region of the cavity.

Another object of the present invention is to establish a more uniformtime-averaged electromagnetic field distribution in a microwave cavityby simulating the effects of deforming or moving a wall of the cavitywith a rotatably mounted mode stirrer assembly.

A further object of the present invention is to establish a more uniformtime-averaged electromagnetic field distribution throughout a largevolume microwave cavity.

Still another object of this invention is to provide a simplyconstructed and easily operable rotatably mounted mode stirrer assemblywhich produces a more uniform time-averaged electromagnetic fielddistribution throughout a large volume microwave cavity.

The present invention is a multimode microwave cavity for subjectingmaterials to microwave energy which accomplishes the foregoing and otherobjects thereby overcoming many of the limitations and disadvantages ofthe prior art microwave cavities. More specifically, in accordance withthe present invention, a microwave cavity has conductive boundary wallsforming a compartment for subjecting materials to microwave energy. Amicrowave generator provides microwave energy at a selected frequencyand is coupled to the cavity to deliver microwave energy thereto wherebyan electromagnetic field is distributed throughout the compartment. Tochange the electromagnetic field intensity distribution in thecompartment, a mode stirrer assembly is employed which comprises aplurality of structures, each having at least one slat of conductivematerial rotatably mounted at an axis of rotation located proximate aconductive wall of the cavity. The structures are positioned within thecavity to have the slats of adjacent structures spaced apart to extendat intervals across the front of the proximate wall. The closer thespacing between the slats of adjacent structures as well as the spacingbetween the slats and the proximate wall, the more nearly the operationof the mode stirrer assembly simulates the change in the geometric spaceof the compartment as would be produced by deforming or moving theproximate wall of the cavity. The mode stirrer assembly simulates almostexactly the eflfect of deforming or moving the proximate wall when thestructures are positioned so that the spacing between the slats ofadjacent structures when in positions of their closest possible approachis not greater than one-quarter wavelength of the microwave energy andthe distance between the slats and the proximate wall of the cavity whenthe slats are in positions of their closest possible approach thereto isnot greater than one-quarter wavelength of the microwave energy providedby the generator.

The uniformity of the time-average electromagnetic field intensitydistribution throughout the compartment is enhanced, particularlythrough the region in which the materials are located for subjection tomicrowaves, by introducing the microwave energy into the cavity andlocating the slats at opposite sides of the cavity. This constructionplaces the material to be subjected to microwaves between the modestirrer assembly and the point at which the microwave energy isintroduced into the cavity.

BRIEF DESCRIPTION OF THE DRAWING The foregoing and other objects andadvantages of the microwave cavity apparatus of the present invention'will become more apparent from the following detailed description andappended claims considered together with the accompanying drawing inwhich:

FIG. 1 is a perspective view with a broken-away portion of oneembodiment of the microwave cavity of the present invention forsubjecting materials to microwave energy.

FIG. 2 is an enlarged front view of a portion of this mode stirrerassembly employed in the microwave cavity taken along line 22 of FIG. 1showing the relative orientation of the slats forming the mode stirrerassembly.

FIG. 3 is an enlarged top view of the mode stirrer assembly employed inthe microwave cavity taken along line 3-3 of FIG. 1 showing the relativeorientation of the slats and proximate cavity wall, with the slats in adifferent position than that shown in FIG. 2.

FIG. 4 is an enlarged end cross sectional view of one of the slats ofthe mode stirrer assembly taken along line 44 of FIG. 3.

FIG. 5 shows a slat of the mode stirrer assembly mounted inside a tubeof low loss dielectric.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, theapparatus of the present invention comprises a multimode microwavecavity 11 having conductive boundary walls of, for example, aluminumjoined together to define a compartment 12 for receiving materials to besubjected to microwave energy in order to, for example, heat thematerials. In the illustrated embodiment, a rectangular cavity 11 isshown which is suited for subjecting materials to microwave energy in abatch process. To place the materials within the compartment 12, thefront wall 13 is provided with a hinged access door 14. The multimodemicrowave cavity 11 also could be a conveyorized type having inlet andoutlet passages allowing the continuous transportation of materialsthrough the compartment 12.

To subject materials placed within the compartment 12 to microwaveenergy, the cavity 11 is provided with an input waveguide feed 16 and iscoupled to a microwave generator 17 by a waveguide transmission line 18joined to the waveguide feed 16. The microwave generator 17 is operatedto provide microwave energy at a certain power, for example, 5 kw., andat a suitable frequency, such as 915 mHz. The cavity 11 is constructedto define a compartment 12 of a size so that a large number of differentelectromagnetic field distribution patterns can be established therein.Microwave energy may be introduced simultaneously into the cavity 11 ata plurality of locations in the walls of the cavity if desired.

To vary the electromagnetic field distribution in the compartment 12 andthereby, for example, effect a more uniform heating of the materiallocated in the compartment, a mode stirrer assembly 19 is provided tochange the geometric space of the compartment 12 as seen by theelectromagnetic field established therein. Referring to FIGS. 1-3, themode stirrer assembly 19 of the present invention includes a pluralityof generally flat rotatably mounted slats 21 of electromagneticreflective material, such as aluminum, located proximate and spaced froma boundary wall 22 of the cavity 11. The slats 21 are positioned atintervals across the front of the wall 22 and extend longitudinallyalong their axes of rotation. In this connection, as shown in FIG. 3,each of the slats has a length along its axis of rotation which isgreater than its Width.

Although the slats 21 can be located proximate any of the boundary wallsof the cavity 11, the most beneficial mode stirring results are obtainedby locating the waveguide feed 16 and slats 21 at opposite sides of thecompartment '12. In the illustrated embodiment, the waveguide feed 16 islocated in the top wall 23 of the cavity 11 proximate the front wall 13and the slats 21 are positioned proximate the rear wall 22 of thecavity.

Each of the slats '21 are journally supported about an axis of rotation24 by shafts 26 which engage bearings 27 at, for example, opposite sidewalls 28 and 29. By supporting the slats 21 in this manner, they extendhorizontally in front of the rear wall 24. However, the slats 21 alsocould be oriented to extend vertically in front of the rear wall 24.Preferably, the slats 21 are supported parallel spaced apart at auniform distance from the rear wall 24 parallel to the bottom Wall 32 ofthe cavity 11.

To drive the rotatably mounted slats 21, one end of the shaft portions26 of each of the slats 21 extends exteriorly of the cavity 11 and iscoupled to a drive motor 33 by a suitable power transmission system 34.In the embodiment of the figures, a link chain transmission system 34 isshown which rotates the slats 21 synchronously and in the samedirection. The slats 21 may be driven in other ways as well. Forexample, the slats 21 could be reciprocated, driven separately, drivenout of synchronism, or driven in opposite directions. To rotate theslats 21, the link chain transmission system 34 includes a drivensprocket 36 fixed at the end of the shaft portion 26 of each of theslats 21. The sprockets 36 are driven together by a first link chain 37.The drive motor 33 is coupled to drive the link chain 37 by a drivingsprocket 38 fixed to the drive shaft 39 of the motor 33 and coupled by asecond link chain 41 to drive a second driven sprocket 42 fixed to anextension of one of the shaft portions 26.

Referring to FIG. 2 in detail, the width, W, in the directionperpendicular to axis of rotation 24 of each of the slats 21 is at leastone-quarter and preferably about onehalf of the wavelength of theapplied microwave energy. The width of each of the slats 21 may belarger than onehalf wavelength if desired. To closely simulate thechange in the geometric space of the compartment 12' that would beproduced by deforming or moving the rear wall 22 of the cavity 11, theslats 21 are located within the cavity so that the spacing, S, betweenadjacent slats when in the position of the closest possible approachbetween the adjacent slats is less than one-quarter wavelength of theapplied microwave energy. In the embodiment illustrated, the one-halfWavelength wide slats 21 are synchronously rotated about theirlongitudinal axis 43 so that at all times each of them is in the someorientation relative to the rear wall 22 of the cavity 11. Therefore,the slats 21 are mounted at locations spaced apart one-half wavelengthplus the spacing S, which in the embodiment illustrated is aboutone-tenth wavelength. The spacing S could be larger than one-quarterwavelength of the applied microwave energy. However, as the spacing S isincreased, the mode stirring effect produced by the mode stirrerassembly 19 becomes less similar to that produced by deforming or movingthe rear wall 24 of the cavity 11.

With reference now to FIG. 3 in detail, the slats 21 preferably aremounted within the cavity 11 so that the distance D between the slatsand the proximate rear wall 24 when the slats are in the position of theclosest possible approach to the rear wall 24 is less than one-quarterwavelength of the applied microwave energy. In the illustratedembodiment, the distance D is about one-tenth wavelength. With the slats21 located as shown in FIGS. 2 and 3, the operation of the mode stirrerassembly 19 results in a change in the geometric space of thecompartment 12 as seen by the electromagnetic field which essentially isthe same as that which is produced by deforming ormoving a wall ofcavity 11. However, as explained hereinbefore, the mode stirrer assembly19 of the present invention can produce a more rapid change in thegeometric space of the compartment 12, hence, in the distribution of theintensity of the field, more easily than can be produced by deforming ormoving a wall of the cavity 12.

Referring now to FIGS. 3 and 4, an embodiment of the slats 21 of themode stirrer assembly 19 is illustrated which is particularly rugged andfacilitates rotating the mode stirrer assembly 19 under conditions ofdynamic stability. Each slat 21 comprises four flat stainless steelsegments 44 joined at their opposite ends 46 and 47 by welds 48 aboutthe shaft portion 26. As illustrated in the figures, the slats 21 areshown as being rectangular. However, to prevent arcing between the ends52 and 53 of the slats and the proximate cavity walls, the ends may bearcuate or wedge-shaped. In any case, in practice it is preferred thatthe opposite longitudinal edges 54 and 56 be straight and parallel. Asassembled, the segments 44 of each slat 21 are joined to the shaftportion 26 at their ends 46 by welds 48. The shaft portion 26 axiallyextends the entire length of the slat 21 with two sections 49 and 51defined by the joined segments 44 extending from opposite sides of theshaft portion 26. The axially extending shaft portion 26 makes the slat21 rugged. Since the axis of rotation 24 of each of the slats 21 iscoextensive with its longitudinal axis 43, the mode stirrer assembly 19will be dynamically stable, thereby, greatly simplifying mounting theassembly for rotation.

The mode stirrer assembly 19 illustrated in the figures is shown asincluding a plurality of slats 21 having two sections 49 and 51. Theslats 21 could be provided with additional sections mounted, forexample, about the axis 43 to extend from the shaft portion 26.

One embodiment of the multimode microwave cavity of the presentinvention constructed in accordance with the FIGS. 1-4 for operation ata microwave frequency of 915 mHz. has the following specification. Thecompartment 12 is 54 inches wide, 42 inches high and 92 inches deep.Four slats 21 are rotatably mounted within the cavity 1 1 centrallybetween the side walls 28 and 29 with each slat 21 having a length of 48inches and a width of 6 inches and /2 inch diameter shaft portion 26.Each of the slats 21 is mounted with its axis 43 located 4 inches fromthe rear wall 24 and spaced 7 inches from the axes 43 of adjacent slats21. The slats 21 adjacent the top and bottom walls 23 and 32'are mountedwith their axes 43 located 4 inches therefrom.

In FIG. 5, a slat 21 constructed in accordance with the embodiment ofFIG. 4 is shown inserted within a tubular polypropylene shield 52. Thetubular shield 52 is supported at the shaft portion 26 by spokes 53. Thetubular field 52 facilitates cleaning the mode stirrer assembly 19.

Although the multimode microwave cavity of the present invention hasbeen described in detail with reference to a particular embodiment, fromthe description it is apparent that many modifications and variationsare possible without departing from the scope of the invention.Therefore, the present invention is not intended to be limited except bythe terms of the following claims.

What is claimed is:

1. Apparatus for subjecting material to microwave energy having apredetermined wavelength comprising conductive walls forming a multimodemicrowave cavity defining a compartment for receiving said material tobe subjected to the microwave energy, means for introducing microwaveenergy into said cavity at least at one selected location, and aplurality of structures mounted for rotation within said cavity adjacenta wall thereof about axes which are generally parallel to said wall andare spaced at intervals along said wall, each of said structuresincluding at least one slat of microwave reflective material extendinglongitudinally along said axis of rotation and having a lengththerealong greater than its dimension perpendicular to said axis ofrotation, said dimension of said slat perpendicular to said axis beingat least about one-quarter wavelength of the microwave energy.

2. The apparatus according to claim 1 wherein each of said slats isspaced apart from adjacent slats so that the spacing between adjacentslats when in positions of their closest possible approach is notgreater than onequarter wavelength of the microwave energy.

3. The apparatus according to claim 1 wherein said slats and location ofintroducing microwave energy into said cavity are at opposite sides ofsaid cavity.

4. The apparatus according to claim 1 wherein each of said slats isspaced from the proximate conductive wall so that the distance betweeneach of the slats and the proximate conductive wall when the slat is inits position of closest possible approach to the proximate conductivewall is not greater than one-quarter wavelength of the microwave energy.

5. The apparatus according to claim 4 wherein each of said slats isspaced apart from adjacent slats so that the spacing between adjacentslats when in positions of their closest possible approach is notgreater than one-quarter Wavelength of the microwave energy.

6. The apparatus according to claim 1 wherein said proximate conductive,wall is planar, and said slats are rotatably mounted parallel spacedapart.

7. The apparatus according to claim 6 wherein said 7 slats are rotatablymounted at a uniform distance from said proximate conductive wall.

8. The apparatus according to claim 6 wherein each of said slats iselongated and has parallel opposite longitudinal edges, each of saidslats has an axis of rotation coextensive with its longitudinal axis,the spacing between adjacent slats when in positions of their closestpossible approach is not greater than one-quarter wavelength of themicrowave energy, and the distance between at least some of the slatsand the proximate conductive wall when such slats are in their positionof closest possible approach to the proximate conductive wall is notgreater than onequarter wavelength of the microwave energy.

9. The apparatus according to claim 8 wherein said slats and location ofintroducing microwave energy into said cavity are at opposite sides ofthe cavity.

10. The apparatus according to claim 8 wherein each of said slats has alateral dimension of about one-half wavelength of the microwave energy.

11. The apparatus according to claim 1 further comprising means coupledto rotatably drive said slats.

12. The apparatus according to claim 11 wherein said rotatably drivemeans is coupled to synchronously-rotate said slats.

13. The apparatus according to claim 1 further comprising a microwavegenerator coupled to said cavity and operated to provide microwaveenergy at a selected power at said predetermined wavelength.

References Cited UNITED STATES PATENTS 3,218,429 11/1965 Lenart 21910.553,277,580 10/1966 Tooby 21910.55 3,364,332 1/1968 Reftrnark 21910.553,431,381 3/1969 Anderson 21910.55

JOSEPH V. TRUHE, Primary Examiner L. H. BENDER, Assistant Examiner

